Positron Emission Tomography (PET) imaging provides unique functional data that has the potential for supplementing anatomical imaging in defining tumor shapes and locations for radiation therapy treatment planning. PET images take many minutes to acquire and for tumors in the thorax and upper abdomen breathing motion causes three effects that are significant with respect to using PET images for tumor delineation. Firstly is the physical distortion of the tumor caused by the breathing motion which yields an inaccurate shape of the tumor. Secondly, a quantitative determination of the apparent PET intensity that provides the most accurate tumor surface delineation will not be possible. Thirdly, determining the tumor volume with a motion-artifact compromised image will yield an inaccurate result which will provide undesired uncertainty in the clinical outcomes data. We hypothesize that with proper breathing monitoring and mapping of tumor and normal organ breathing motion, a breathing motion-artifact free PET image study can be generated that retains the statistical precision as thought the patient had held their breath throughout the PET scan procedure. Further, the PET image can be generated for any user-selected reference breathing phase. This will be implemented using modern multislice PET/CT systems and a quantitative 5-dimensional (5D) model of breathing motion as a function of breathing air volume (tidal volume) and the rate of breathing (airflow). The patient-specific data for this model will be provided by a breathing-gated 5D-CT scan. The PET scan will be conducted while the patient undergoes the same breathing monitoring (5D PET). A series of PET images will be reconstructed for each phase of breathing and quantitatively warped and added in the reference breathing phase to yield the breathing motion artifact-free PET image study. This work will provide the radiation-oncology clinician with PET images having unrivaled resolution and sensitivity for target definition, leading to the improved efficacy of radiotherapy in the thoracic and abdominal regions.